This invention relates to a burner control system of the type disclosed in applicant's U.S. Pat. No. 3,092,146. The system illustrated in such patent comprises a primary pump for drawing oil from a supply line, in a quantity in excess of the capacity of a secondary pump, into a reservoir wherefrom the independent secondary pump communicates such oil from the reservoir, through a viscosity valve and into a burner for firing purposes. Such system functions satisfactorily to supply oil at a substantially constant flow rate for most burner applications for burning designated "heavy oils" at pumpable viscosities varying from approximately 50 to 500 Saybolt Universal Seconds or 11 to 50 Saybolt Furol Seconds. Various local regulations have now dictated that such systems must be capable of compensating for oil having a wider range of viscosities extending into the lighter bands, as low as 30 Saybolt Universal Seconds.
As shown in FIG. 2 of the drawings which illustrates a standard ASTM viscosity chart for fuel oil, such viscosities may range from the relatively light "No. 2 max." to the heavier "No. 6 max.". Although the secondary pump is of positive displacement type for pumping a constant volume of oil to the viscosity valve, a certain amount of slippage may occur therein as a result of the differential pressure required on the respective sides of the secondary pump. There may be intolerable leakage when handling oil having viscosities substantially below customary values.